RF amplifier circuits are used in a wide variety of applications. High power transistors such as LDMOS (laterally diffused metal oxide semiconductor) transistors are commonly used to form RF amplifiers. These transistors are typically packaged in a universal device package that can be easily connected to an external component, such as a printed circuit board (PCB).
Doherty power amplifiers are becoming increasingly commonplace in RF applications due to their high efficiency over a wide bandwidth. A Doherty amplifier configuration includes a main amplifier and a peaking amplifier. The main amplifier operates at high efficiency and provides RF signal amplification during most operating ranges. When additional power is required, the peaking amplifier turns on. Packaged Doherty amplifier configurations typically include input and output match networks integrated within the device package to feed the RF signal into the main and peaking power transistors and to combine the RF power generated from the main and peaking power transistors. The input and output match networks are also typically tuned to a fixed impedance match value (e.g., 50 ohms).
In a Doherty amplifier, the input impedance of the peak amplifier (which is biased in class C) will change with RF power drive level. This change in impedance can detune the input matching of the main amplifier, leading to unfavorable am-am and am-pm performance, or even reliability and ruggedness problems.
One way to mitigate detuning of the input matching network is to externally connect (i.e., outside of the package) a hybrid coupler. The hybrid coupler isolates the RF signal between inputs of the main and peaking amplifiers. However, one drawback of this technique is that it requires the input of the main and peaking power transistors to first be matched to 50 ohms before connection to the hybrid coupler, and this limits the frequency bandwidth of the Doherty amplifier. Additionally, externally connectable hybrid couplers are only available with a fixed phase differential e.g., 90 degrees, 180 degrees etc. In many cases, it is desirable to adjust the overall phase differential to the optimum value (e.g., slightly more or less than 90 or 180 degrees). The only way that this can be done is to interpose phase delay lines (e.g., inductors, capacitors, PCB transmission lines, etc.) between the hybrid coupler and the packaged Doherty amplifier, which increases cost and complexity of the device. Use of external hybrid couplers therefore limits how small and cost effective the Doherty amplifier board can be made.
In another configuration, the external hybrid coupler is omitted and the inputs of the Doherty Amplifier are joined by an external 90 degree phase line. However, without the hybrid coupler, there is poor isolation between the main and peak amplifiers. This leads to performance and reliability issues. In addition, am-am and am-pm performance becomes very sensitive to manufacturing variations in the main and peak transistors, thus leading to poor yield.